Field of Invention
The present invention refers to an electric discharge machine and to a pulse generator module, in particular to the arrangement of the pulse generator module in such an electric discharge machine.
Discussion
An electric discharge machine (EDM) comprises essentially a machine frame, a dielectric conditioning unit, a control unit and a pulse generator. According to the current practice, the pulse generator is located in a cabinet, generally with the control unit and motor control boards, power supplies, etc. Said cabinet is generally located behind or at the side of the machine frame. The pulse generator boards and the other boards are arranged in racks guided in rails, with the printed circuit boards (PCB's) parallel to each other, side by side. In this way the generator is easily accessible during the assembly and cabling but also to service personnel and the boards can be replaced in case of necessity. Generator boards are connected with a power supply and are communicating with other boards in the same cabinet. Special shielded generator cables are used to feed the pulses to the electrodes.
It is well known that stray capacitances and inductances affect pulse shape. In particular short high current pulses with high current gradient (as typically used in WEDM and micro EDM) are flattened, that is current gradient and peak current are reduced and pulse duration is extended. To avoid these negative effects, the entire circuitry and cabling are constituted in consideration thereof.
U.S. Pat. No. 5,166,489 to HUFF discloses the so called gap boxes, which are arranged close to the spark gap in wire electric discharge machines. The gap box is housing the isolation diodes, which represent a portion of the power supplying circuitry that supplies the desired waveform of pulses to the wire electrode. The gap box housing has comparably small dimensions; lower and the upper gap boxes are sometimes arranged respectively in the upper and in the lower wire guiding arm. The lead distance from gap box to current feeder is only in the order of 200 mm. The gap box sometimes comprises a cooling circuit to dissipate the heat produced by the gap box circuitry and to avoid thermal distortion of machine frame and other structural components. The use of these gap boxes have led to great improvement of the quality of the surfaces machined by WEDM.
Japanese Patent Document No. 4-250920A to INOUE discloses generator modules provided under the workpiece mounting base, which are stored in a watertight casing and water-cooled by means of the machining liquid directly via the casing of these generator modules. The current supply to the workpiece side is directly executed from the generator module to the mounting base, without long cables. INOUE provides very high integration, unfortunately the space available under the workpiece mounting base is very limited, and there is practically not enough space to house the generator boards according to current designs. Considering that generator casings and cable couplings must be perfectly watertight but still accessible, it becomes apparent that this solution is not applicable with current technique.
U.S. Pat. No. 6,080,953 to BANZAI discloses a pulse generator module arranged in proximity of the spark gap, wherein processing fluid is used for direct cooling of the heat generating elements. Several variants are displayed; the enclosure of the generator is immersed or exposed to the machining liquid so as to obtain a direct cooling effect by means of the cooling liquid. In this way, separate cooling of the generator module becomes unnecessary. However, with this approach the machining liquid in the processing area is the same as the generator cooling liquid, which is particularly critical during the main cut. The waste heat of the generator module is fed into the work tank, making it even more difficult to keep the temperature of the working fluid at a stable value (prerequisite for having a precise machining of a workpiece). By adapting the BANZAI system, it would be necessary to limit respectively compensate errors due to thermal distortion of workpiece and machine body caused by the additional waste heat produced by the generator module.
Further, EP 1 749 609 to BÜHLER discloses an EDM that comprises a plurality of configurable modules, for instance a machine control module, a drive module and a generator module. These modules are linked to a node to form a data network, so that they can send and receive data from said node. In this way the modules can be located where needed. For instance, said generator module and a process monitoring module are conveniently arranged in the vicinity of the spark gap. Still, the application of this state of the art requires the use of a specific node and requests a sophisticated network systems, which is technically complex.